The invention relates to a process for the production or recovery of acids from solutions, in particular solutions of such acids containing metal salts, by spray roasting of the solutions and subsequent absorption and/or condensation of the gases formed thereby, the solids which form thereby being withdrawn.
In particular the invention is directed to a process and an apparatus for the production or recovery of nitric acid or of a mixture of nitric acid and hydrofluoric acid from solutions thereof, in particular from solutions containing cations such as Fe, Cr, Ni, Ci, Zr, Al and others. Such solutions are generated during the surface treatment (pickling, etching, chemical erosive shaping) of steels, stainless steels (chromium and chromium nickel steels) special purpose alloys (e.g. nickel-based alloys) and special surface metals (e.g. titanium, zirconium tantalum) as well as the dissolution of such metals or their compounds in the course of metallurgical processes. Accordingly these solutions contain the ions of the abovementioned metals as well as other alloying elements or contaminants which are contained in smaller amounts. These solutions must be discarded after having attained a certain metal content which--dependent upon the pickled alloy--amounts to 20 to 60 g/l of dissolved metal. Usually such solutions are disposed of by neutralisation. Because of the large amounts of sludge formed thereby as well as the nitrate loads of the effluents this gives rise to an appreciable environmental problem. Moreover, there results for the plant also a major expenditure in respect of the purchase of makeup acid as well as for neutralising chemicals. Accordingly, for several years now, the literature has been describing processes which permit at least a partial recycling of the aforesaid chemicals.
The disadvantage of all those processes is that most of these permit the processing of only one particular waste solution, usually a solution of Fe, Cr and Ni in a mixture of hydrofluoric acid and nitric acid as arises in the treatment of stainless steels. Moreover, in many of those processes the metals contained in the solution are again converted into neutralisation sludge, whereby the dumping problem is not solved.
Generally speaking the aforesaid processes can be subdivided into the following groups.
1. Distillation Processes
In such processes sulphuric acid is added to the spent pickling solution and the volatile acids HNO.sub.3 and HF are distilled off. The metal content is converted into a mixture of sulphates which must be processed further. One particular process employs for that purpose a precipitation of the iron in the form of jarosite and the removal of the nickel as the hydroxide. Besides the cumbersome processing methods--several evaporation, crystallisation and filtration stages are required--the chemicals consumption is a further disadvantage. This process cannot be applied to titanium containing solutions, because the titanium sulphates do not crystallise readily.
2. Crystallisation Processes
In this type of process the solution is evaporated down in a special evaporator heated with electric current and then iron fluoride crystals are formed by cooling, the crystals being filtered off. The filtrate constitutes part of the recovered acid, the other part is recovered by thermal decomposition of the fluoride crystals in a rotary kiln in the form of hydrofluoric acid. This process attains a high recovery yield for hydrofluoric and nitric acid, and by roasting off the primarily arising FeF.sub.3 crystals, pure hydrofluoric acids can be recovered, not, however, pure nitric acid. Not all cations contained in the solution are precipitated to the same extent by the crystallisation steps, but preferentially iron. Accordingly, a cyclic pickling and regeneration system may give rise to an accumulation of minor elements (Cr, Ni). The process can be applied to certain solutions other than those of Fe/Cr/Ni, e.g. to solutions containing Zr, but not Ti.
3. Extraction Processes
These processes comprise a plurality of process steps, namely
iron extraction (with diethyl hexyl phosphate in paraffin), PA1 recovery of the extraction agent by the addition of NH.sub.4 HF.sub.2, PA1 crystallisation of (NH.sub.4).sub.3 FeF.sub.6, PA1 decomposition of the (NH.sub.4).sub.3 FeF.sub.6 at 500.degree. C., PA1 calcining of the product at 800 7.degree. C., PA1 acid extraction (tributyl phosphate in paraffin) PA1 neutralisation of the residual solution left behind.
Apart from the complicated procedures of this process, it entails a continuous risk of effluent contamination by the extraction agent as well as a fire hazard due to the flammable solvents. The process has been described expressly only for Fe--Cr--Ni solutions. Solutions which contain Ti or other metals cannot be treated in this manner.
4. Ion Exchange Process
In this process the free acids (but not the portion bound to the metal) are sorbed onto an ion exchanger resin and are then recovered by elluting with water. The metals are released in the form of a metal salt solution which must be neutralised. This is therefore not in fact a true regeneration process but merely an upgrading of a partly spent solution. The metal contents which have to be neutralised are present at the same level as in mere neutralisation.
5. Dialysis Processes
These may be operated either as a pure diffusion dialysis or as an electro-dialysis. In the diffusion dialysis there takes place, in analogy to the ion exchange processes, merely a separation of free acids and metal salts, giving rise in principle to the same problem. In the electrodialysis the solution as a whole is first neutralised with caustic potash whereafter the resulting neutral solution of potassium nitrate and potassium fluoride is separated by means of special membranes into acid and caustic potash with the application of electric current. The recovery yields are good. However, the electricity consumption is high and the membranes are very sensitive.
For the complete recovery of hydrochloric acid as is used for the pickling of carbon steel, the spray roasting process has been used for years. In that process the spent pickling solution is sprayed into a reactor where it is contacted with hot gases generated by a burner projecting into the reactor. This results in a complete decomposition of the solution: EQU 2 FeCl.sub.2 +2 H.sub.2 O+1/2O.sub.2 =Fe.sub.2 O.sub.3 +4 HCl
The metal content of the solution is converted into metal oxide, and the HCl gas which is generated is reabsorbed in water in the form of hydrochloric acid. In order to attain the above described decomposition, temperatures typically in the range of about 500.degree. to 1000.degree. C. must be attained in the reactor. This and similar processes have to date been used only for the recovery of hydrochloric acid. A prejudice existed against the application of this process to the recovery of other acids, e.g. hydrofluoric or nitric acid, because at the aforementioned temperatures a substantial decomposition of the nitric acid into NO was to be expected which cannot be absorbed readily. Moreover, at those temperatures corrosion problems were to be expected.